1. Field of the Invention
The invention relates to a method of transforming plant fiber material into produce saccharide.
2. Description of the Related Art
Effective use of saccharide as food or fuel has been proposed and is being put into practice, the saccharide being mostly glucose and xylose and produced from cellulose or hemicellulose by transforming plant fiber material, such as squeezed sugarcane residues (bagasse) or wood chips. In particular, biomass energy technology is drawing attention, in which the saccharide obtained by transforming plant fiber material is fermented to produce alcohol, such as ethanol being used as fuel. In Japanese Patent Application Publication No. 8-299000 (JP-A-8-299000), Japanese Patent Application Publication No. 2006-149343 (JP-A-2006-149343), Japanese Patent Application Publication No. 2006-129735 (JP-A-2006-129735), and Japanese Patent Application Publication No. 2002-59118 (JP-A-2002-59118), for example, various methods of producing saccharide, such as glucose, by transforming cellulose or hemicellulose, are proposed. JP-A-8-299000 describes a method of hydrolyzing cellulose with the use of hydrochloric acid or sulfuric acid such as dilute sulfuric acid or concentrated sulfuric acid. A method in which cellulase is used (JP-A-2006-149343), a method in which a solid catalyst, such as activated carbon or zeolite, is used (JP-A-2006-129735), and a method in which pressurized hot water is used (JP-A-2002-59118) are also available.
In the case of the method in which cellulose is transformed with the use of acid, such as sulfuric acid, however, it is difficult to separate the acid and saccharide. This is because acid and glucose, which is the main ingredient of the transformation product, are both soluble in water. Removal of acid by neutralization or ion exchange is not only troublesome and costly, but it is also difficult to completely remove acid because acid may remain in the process of fermentation for ethanol. As a result, even when pH is optimized in view of activity of yeast in the process of fermentation for ethanol, concentration of salt becomes high, which results in reduction in activity of yeast, which in turn results in reduction in fermentation efficiency.
In particular, when concentrated sulfuric acid is used, it is very difficult and very energy consuming to remove sulfuric acid to the extent that yeast is not deactivated. On the other hand, when dilute sulfuric acid is used, it is relatively easy to remove sulfuric acid. However, it is necessary to transform cellulose under high temperature conditions, which is energy consuming. In addition, the acid, such as sulfuric acid and hydrochloric acid is very difficult to separate, collect and reuse. Thus, use of these acids as a catalyst for producing glucose is a cause of increasing the costs of bio-ethanol.
In the case of the method in which pressurized hot water is used, it is difficult to adjust the conditions, and it is therefore difficult to produce glucose with stable yield. In addition, according to the above method, even glucose is transformed to cause reduction in the yield of glucose, and moreover, the activity of yeast is reduced due to the transformation product, which may result in suppression of fermentation. Furthermore, the reactor (supercritical processing apparatus) is expensive and is low in durability, and therefore, this method is problematic also in view of costs.
Meanwhile, widely used catalysts include a cluster acid catalyst, such as heteropoly acid. In Japanese Patent Application Publication No. 2006-206579 (JP-A-2006-206579), for example, a method of manufacturing ester levulinate is described, which carbohydrate and alcohol are reacted under the presence of heteropoly acid. In a method described in WO95/26438, a cluster acid catalyst, in the form of aqueous solution of 0.001 to 0.20 M, is used in the process of removal of lignin from wood pulp and the process of bleaching the wood pulp.